The current industrial carbon electrodes are typically manufactured by blending petroleum coke particles (the filler) with molten coal tar pitch (the binder) and extruding the resultant mix to form the "green electrode". The green electrode is then baked at approximately 1300.degree. C. These heat treating processes transform the green body from approximately 95% carbon content to greater than 99% carbon. During the heat treating process, some of the organics are destructively distilled or vaporized and others decomposed, resulting in carbon deposition in the electrode. As the vaporized materials exit the body of the electrode they channel through its walls producing a porous structure. The result of this inherent porosity is reduced density, and reduced current carrying capacity.
In the production of carbon electrodes, the carbon industry produces electrodes as large as 28 inches in diameter by 10 feet long for use in electric arc furnaces. These electrodes are used for example to carry large quantities of current in steel melting processes. The characteristics of a desirable carbon electrode are:
1. high density PA1 2. high modulus of elasticity PA1 3. high electrical conductivity PA1 4. high flexural strength PA1 (a) increased yields PA1 (b) reduced sulfur content PA1 (c) increased density PA1 (a) sulphur content less than 0.5 wt.% PA1 (b) a density at 77.degree. F. greater than 1.28 grams per cc PA1 (c) a Cleveland Open Cup flash point greater than 200.degree. C. PA1 (d) an in-situ coking value of 32 wt.% PA1 (e) Rate of pick-up of impregnant by the electrode comparable to that of a petroleum pitch and exceeding that of other coal tar based pitches. PA1 (1) the quinoline insoluble (QI) content must be less than 0.05 weight percent as determined by ASTM D-2318-76; and PA1 (2) the distillation residue according to ASTM D246-73 is greater than about 25%, with about 60% preferred.
To reverse the undesirable effect of channeling, inherent porosity and reduced current carrying capacity the electrode is impregnated with an impregnating pitch which must have properties particularly suitable for this purpose.
Coal tar pitch has historically been used as the impregnant because of its relative high density and carbon content as compared to petroleum pitch. However, technological improvements in manufacturing carbon electrodes have led to reduced porosity and pore size of the green bocy. As a result, impregnating pitch of lower solid content must be used. Ordinary coal tar based pitch cannot meet this requirement. While the market is currently dominated by petroleum based pitch, this material also has certain definite drawbacks. Moreover, it is to be understood that solid content of a pitch is only one indicator of pitch quality; the ultimate measure of quality pertains to penetration rate (high rates are desired) and ultimate yield of coke after rebaking.
The solids content of a pitch is normally measured in weight percentage of the pitch and is determined by ASTM D2318-75 in terms of "quinoline insoluble" (QI).
At this point it is significant to note that the term "pitch" is applied to a wide range of compositions and there is a distinct difference between pitches used for various purposes. With particular reference to electrode production "pitch" may be used in at least three different ways.
1. Pitch can be coked to form "pitch coke" which is pulverized, sized and used as filler. Currently, most coke filler is produced from petroleum (as noted above). The manufacture of "pitch coke" from pitch produced by oxidizing coal tar at high temperatures is also known. However, it is to be noted, that pitch used as precursor of "pitch coke" has no "low solids" content requirement as does an impregnating pitch which is the material with which the present invention is concerned.
2. Pitch can be used as a binder or cement to hold the carbon electrode during forming and baking. This application requires a coal tar pitch with its inherently high quinoline insolubles (QI) content. The significance of quinoline insolubles in binder pitches is described, for example, in D. R. Ball, "The influence of the type of Quinoline Insolubles on the quality of coal tar binder pitch" (Carbon 16, page 205 [1978]). It is generally agreed, that the solids content of binder pitches is determined by the "QI" test. It should also be noted that previous use of high-temperature oxidation of carbonacious materials (petroleum, coal tar, and oils) to form pitches suitable for electrode production were directed towared the production of binder pitches, and pitches for pitch coke, not for impregnant pitches. These prior art pitches usually had a QI content of the order of 14 percent.
3. While reference to "impregnating pitches" for use in electrode production have been made, this application requires a pitch with distinctly "low solids" content. A discussion of the use of impregnating pitch and the physical properties of pitches used as both binders and impregnants may be found in Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 4, pg. 168, 181-183. The major difference between binder pitches and impregnating pitch can be seen from inspection of the "quinoline insoluble" line of Table 3, at page 168 of that reference.
______________________________________ TYPICAL COAL TAR BINDERS IN CARBON AND GRAPHITE MANUFACTURE SOFT MEDIUM HARD IMPREGNATING PITCH PITCH PITCH PITCH ______________________________________ QI % 12 12 15 5 ______________________________________
The QI of binders is significantly higher than the QI of impregnants. As shown, the QI content of a regular coal tar based impregnant is 5 wt%.
In recent years, the quality of electrode has improved and the criteria for specifying the impregnating pitch has become more stringent. Impregnating pitch containing 5 percent QI is no longer satisfactory. This is the reason petroleum based pitch displaced coal tar pitch in this application.
The current industrial standard is a petroleum based pitch which contains &lt;0.5% QI. The coal tar pitch of the present invention also contains QI &lt;0.5%. Previously no one has demonstrated the feasibility of producing high quality impregnating pitch based on coal tax oxidation.
An important characteristic of petroleum based impregnating pitch resides in the fact that it possesses a low solids content over regular coal tar pitch. This equates to greater productivity in that it takes less processing time to perform an impregnation. However, petroleum pitch suffers from the disadvantages of low density, high sulfur and low in-situ coking value. In-situ coking value rrefers to the actual yield of carbon in the electrode after baking as compared to the quantity of pitch originally "picked-up" during the impregnation process. For example, suppose an electrode is impregnated, and using "before" and "after" weights, it is determined that the electrode "picked-up" 100 pounds of impregnating pitch. This pitch is transformed to carbon by baking. During baking, low boilers are distilled from the pitch which results in a yield loss. The "before" and "after" weights for the baking process are used to determine the quantity of pitch remaining in the electrode as carbon. Thus, if the electrode after baking weighs 30 pounds more than "before" impregnation, then the in-situ coking value is 30/100=30%.
Typically, the specific gravity at 25.degree. C. of a petroleum impregnating pitch is 1.24 and the specific gravity of a coal tar pitch is 1.30. This difference would equate to a 5% increase in "pick-up" for any impregnation step. It should also be noted that sulphur is an undesirable constituent of pitch because its presence results in an air pollution risk during baking and also produces "puffing" or an undesirable decrease in density phenomenon which can occur during graphitization. It is thus seen that a need exists for the provision of an improved pitch particularly characterized by low solids content, increased in-situ coking value and improved penetration and penetration rate.